1. Field of the Invention
The present invention relates to circuitry for current-level monitoring and for generating a control signal whenever the monitored current level reaches one of several predetermined threshold values. More particularly, the present invention relates to circuitry used for protecting a rechargeable battery against both excess charging currents and excess discharging currents and to isolating such a battery during the time that the battery is not required to power a load. More particularly yet, the present invention relates to current-monitoring circuitry that provides an improvement in the precision with which predetermined current threshold levels can be sensed, without a concomitant increase in the average power demand. Most particularly, the present invention relates to current-monitoring circuitry that remains in a low-power, coarse-precision mode until the current level being monitored approaches a predetermined threshold, at which point the current-monitoring circuitry of the present invention shifts into a high-precision mode, where it remains until either a disconnect (trip) level is reached or the current level falls back below the threshold, and the circuitry shifts back into its coarse mode.
2. Description of the Prior Art
Rechargeable battery technology has gained widespread attention with the increasingly widespread use of portable electronic equipment such as laptop computers, portable CD players, camcorders, and cellular phones. As portability demands grow along with system complexity, the pressure for battery technology improvements increases. At present, the battery technology forefront is focused on batteries made up of lithium-ion (Li+) cells, because of the great improvement in specific energy (energy/mass) availability that such batteries offer over the older nickel-based battery technology. Unfortunately, the improvement the Li+ batteries provide in specific energy is somewhat offset by the susceptibility that these new batteries display to electrical over-stress. This electrical over-stress includes overcharging and over-discharging the battery and excess current to or from the battery. This susceptibility means that Li+-based battery packs must be equipped with circuitry to protect the battery against these stresses. There must be voltage-sensing protective circuitry that will disconnect the battery whenever the voltage across it has fallen to a critical value (e.g., while the battery is powering a load) and whenever the voltage across the battery has risen to a (different) critical level (e.g., while the battery is being charged). Similarly, there must be current-sensing protective circuitry that will isolate the battery whenever the charging current or load-driving current reaches a predetermined high value. The present invention is directed to the latter type of protective circuitry, that designed for protecting the battery against excessive current, be it charging or discharging. (Although the current to be monitored is referred to as a current that is either charging the battery or used to drive a load, it is to be understood that the currents of interest are not limited to these. For example, the battery especially needs to be protected against the extremely high discharge current that occurs when the load is shorted out and also against the extremely high charging current that can result from an charging source being applied to the battery.)
There is a premium placed on the accuracy with which the current level is measured. If the current-monitoring technique is known to have a large uncertainty, then in order to ensure that the battery is protected, it will be necessary to be very conservative in setting the disconnect threshold. E.g., if it is known that I.sub.crit is the maximum current level that can be drawn from the battery without damage to the battery, and it is further known the circuitry monitoring the current level can only determine that level within .+-..DELTA., then the protection circuit will have to be designed so that it disconnects the battery from the load whenever the current being drawn from the battery reaches a level I.sub.crit -.DELTA.. Depending on the size of .DELTA., this can result in a large number of unnecessary disconnects, and all of the inconvenience that results, for example, from the abrupt powering down of a camcorder or a laptop computer while it is in use. An additional detriment arising from the crudeness with which the prior art can track the battery current (charging or discharging) is that to allow for possibly large battery currents various of the semiconductor devices in the circuit, especially power transistors, must be over-designed to be on the safe side. In particular, metal-oxide-semiconductor field-effect-transistors (MOSFETs) must be designed to have a larger area than would normally be necessary, just to provide for the contingency of an abnormally high current due to the crude protective circuitry greatly underestimating battery current. Having to allocate additional circuit real estate (as the result of inaccurate current monitoring) is a significant drawback when circuit space is at a premium, as is normally the case with portable equipment.
The usual way of monitoring a current level involves converting the current into a corresponding voltage, and then monitoring that voltage. If, as in the present case, the objective is to cause some action to be taken when the monitored current reaches a critical value, then a comparator can be used to compare the corresponding voltage with a predetermined threshold voltage (reference voltage) corresponding to the critical current. Although many variations can be made on the comparator inputs in order to determine exactly which pair of voltages the comparator is to compare, the comparator's basic function is to provide a binary output, the instantaneous value of which depends on whether the "test" voltage is greater than or less than the reference voltage. The comparator sensitivity is a measure of how small of a differential between the reference voltage and the test voltage is necessary to cause a shift from one output state to the other. All other things being equal, this sensitivity will be proportional to the current driving the comparator. The cost of increased comparator sensitivity is increased power consumption. The other limit on accuracy and precision is the resistance used to convert the current-to-be-monitored into a voltage. Two major sources of error to be considered are (1) the temperature dependence of that resistance (2) the manufacturing vagaries that lead to variations from one chip to the next of the as-manufactured resistance in comparison with the design resistance. Depending on the type of device used to provide the resistance, yet other sources of inaccuracy or imprecision may arise.
In the prior art current-monitoring battery-protective circuits, the resistance used to convert the battery current to a voltage is that of a MOSFET in its "on" state, that is, R.sub.on, the source/drain resistance of a conducting MOSFET. See, for example, FIG. 1 (prior art) for a schematic illustration of this. The block labeled "IC" contains the comparator circuitry used to isolate the battery--represented schematically as the single cell "B"--when the discharging current reaches a critical level. (It appears that none of the prior art devices provided for monitoring battery-charging current, but rather just battery-discharging current.) FIG. 2 shows that the prior art actually uses two power MOSFETs, M1 and M2, connected drain-to-drain. Through circuitry--not shown--leading to their respective gates the prior art turns on one or the other of these MOSFETs. The back-to-back diodes, D1 and D2, shown in FIG. 2 shunting the two MOSFETs are the body diodes associated with the respective transistors. Since it possible to fabricate the two MOSFETs to be so close to one another as to be practically identical, the current-to-voltage conversion factor--R.sub.on, the source/drain resistance of the conducting MOSFET--will be the same regardless of polarity of the current and thus it would have been straightforward to take the extra step so as to be able to use this type of circuit to monitor and protect against charging currents as well as discharging currents.
There are several drawbacks to the prior-art approach to the current-monitoring task. One is the amount of power that must be used in the comparators in order to have an accurate comparison between the voltage drop across R.sub.on and the predetermined threshold voltage. In addition fine comparators, by their nature, take up significant space on the chip. This is a serious problem, since, in the applications of interest here, there is a demand for ever-smaller chips. One approach within the context of the prior art that addresses both of these problems--excess power and space requirements--is to use coarse comparators instead of fine. This introduces problems that are even worse, since the circuit needs to compensate for the relative lack of sensitivity in determining the magnitude of the current being monitored. The fact is that no comparator is ideal; it is only the ideal comparator that responds (by changing its binary output) as soon as a monitored voltage becomes greater than the reference voltage by an infinitesimal amount. Real comparators will change output state when the monitored voltage exceeds the reference voltage by an amount .DELTA.. Thus, to perform its protective function, disconnecting the battery when the current reaches a critical threshold, I.sub.crit, i.e., when the monitored voltage reaches a corresponding critical voltage Vcrit, the comparator must be set to disconnect the battery when the monitored voltage reaches a level V.sub.crit -.DELTA.. A "fine" comparator will require a relatively small .DELTA. and a "coarse" comparator will require a relatively large .DELTA..
The drawbacks of working with a relatively large A (coarse comparator(s)) is that in order to ensure that the battery is protected it will in general have to be disconnected at currents far below those that would damage the battery, as discussed earlier.
In addition to having to choose between fine comparators, with their high space and power demands, on the one hand and insensitive coarse comparators on the other, the prior-art circuits suffer from temperature dependence and Vgs dependence during operation, because of the variation of the MOSFET resistance, Ron, used as the sensing resistor. This further increases the ".DELTA." safety buffer that must be built into the circuit, thereby increasing the number of unnecessary shut-downs.
Another problem, though of lesser importance, with the prior art battery-protective circuit is the fact that the base value of Ron will vary from chip to chip due to manufacturing vagaries. This fact requires yet an additional increment to be added to the safety buffer. In the prior-art circuit the totality of these effects can result in deviations of as much as 30% between the design R.sub.on and actual R.sub.on.
Although the excess-current concerns have been described up to this point as if the safe range was the same regardless of polarity (i.e., regardless of whether the current was charging the battery or powering the load), this is not always the case. It is occasionally useful to be able to set a different disconnect threshold for battery-charging currents than is set for battery-discharging currents. The prior-art designs, in addition to being burdened with a less-than-desirable means of monitoring current, also appear to be limited in selecting different disconnect levels for the two current polarities. It would appear that within the context of the prior art, the only way in which these different levels can be set is to use two different comparator circuits, with the additional space and power demands that that entails.
Of course, one is not limited to dependence on the resistance across a conducting MOSFET. It is possible to use passive elements, the resistances of which can be tightly controlled, especially when they are provided with a trimming mechanism. Thus, a more accurate current-monitoring-and-response circuit can be introduced. Unfortunately, it cannot be introduced as a replacement to the less accurate monitor described above; the power MOSFETs and associated circuitry are necessary for, among other purposes, the isolation of the battery from the rest of the circuit when conditions call for this isolation. Consequently, adding the additional monitor in order to refine the current control places additional current demands on the battery. Although these additional demands are minuscule by most standards, involving only a fraction of a microamp of additional current to be drawn from the battery, it is quite significant in the present context, where the total current demand on the batteries will be only a few microamps.
Therefore, what is needed is a current-monitoring circuit that more exactly measures current, a circuit that functions with less dependence on operating temperature and gate/source voltages than does the present current-monitoring circuitry. What is also needed is such current-monitoring circuitry that can easily establish and act upon different disconnect-threshold levels for charging and discharging currents, respectively. What is also needed is such a circuit that is independent of manufacturing vagaries with respect to MOSFET resistances. Finally within this context, what is needed is such current-monitoring circuitry that does not significantly increase the power demand over that required by the present current-monitoring circuitry used to protect rechargeable portable batteries.